Besides the official release notes in Histo's answer, Wikipedia also has an article listing all of the different Ubuntu releases, screenshots of each, mentions of major new features and changes, and tables listing each release's life span and certain package version numbers. This can be useful for seeing more of Ubuntu's historical highlights at a glance.

This doesn't quite answer the question, but in future I'd recommending reading OMG!Ubuntu because all the major changes are written about before the next release, and the articles are very well written. The articles about specific releases are tagged using the release codename, for example http://www.omgubuntu.co.uk/tag/quantal has news about

I think the main difference is in support and stability. Ubuntu 12.04 is more stable than Ubuntu 11.04 and 11.10 because it received several bug fix. Ubuntu 12.04 is a Long Term Support (LTS) release (until 2017-04) but Ubuntu 11.04 and 11.10 support is finished soon (2012-10 and 2013-04)